The present invention is directed to a method and composition for use in inhibiting the formation and deposition of coke on surfaces during the elevated tempature processing of hydrocarbons. Coke disposition is generally experienced when hydrocarbon liquids and vapors contact the hot metal surfaces of the processing equipment. While perhaps not entirely technically understood, because of the complex makeup of the hydrocarbons upon elevated temperatures and contact with hot metallic surfaces, the hydrocarbons undergo various changes through either chemical reactions and/or decomposition of various unstable components of the hydrocarbon. The undesired products in many instances include coke, polymerized products, deposited impurities and the like. Whatever the undesired product that may be formed, the result is the same, i.e., reduced economies of the process. If these deposits are allowed to remain unchecked, heat transfer, throughput and overall productivity are detrimentally effected. Moreover, downtime is likely to be encountered due to the necessity of either replacing and/or cleaning of the affected parts of the processing system.
While the formation and type of undesired products are dependent upon the hydrocarbon being processed and the conditions of the processing, it may generally be stated that such products can be produced at temperatures as low as 100.degree. F. but are more prone to formation as the temperature of the processing system and the hydrocarbon reach levels of 600.degree.-1300.degree. F. At these temperatures, coke formation is likely to be produced regardless of the type hydrocarbon being charged. The type coke formed, i.e., amorphous, filamentous or pyrolytic, may vary somewhat; however, the probability of the formation of such is quite high.
As earlier stated the present invention is directed to methods and chemicals for use in the retardation of coke formation in the elevated temperature processes and also to the inhibition of deposition of the coke in the event it is actually formed.
The present invention is particularly effective in hydrocarbon processing systems where temperatures reach levels of 600.degree. to 1300.degree. F. where amorphous and filamentous coke are likely to be formed. Amorphous coke is generally produced in systems where temperatures are less than 850.degree. F. This type coke generally is composed of low molecular weight polymers, has no definite structure and is sooty in nature. Above 850.degree. F., filamentous coke is generally encountered. This type coke, as the name indicates, takes the form of filaments that appear in some cases like hollow tubes. As opposed to amorphous coke, filamentous coke is not sooty and is hard and graphitic in nature.
Amorphous and filamentous coke formation is customarily found in hydrocarbon processing systems such as delayed coking processes (temperature 900.degree. to 1300.degree. F.); platforming, catalytic reforming and magnaforming processes (900.degree. F.); residue desulfurization processes (500.degree. to 800.degree. F.); hydrocracking processes (600.degree.-1,100.degree. F.), visbreaking processes (800.degree.-1000.degree. F.), cracking of chlorinated hydrocarbons, and other petrochemical intermediates at similar temperatures.
Pyrolytic coke is produced in olefin manufacture where pyrolyses of gaseous feed stocks (ethane, butane, propane, etc.) or liquid feed stocks (naphthas, kerosene, gas oil, etc.) are "cracked" by exposing such stocks to temperatures of from 1400.degree. to 1700.degree. F. to produce the desired olefin.
While various treatments have been proposed to eliminate or reduce filamentous coke formation at the 600.degree. to 1300.degree. F. temperatures, none have attained any great degree of success. In the book "Coke Formation on Metal Surfaces" by Albright and Baker, 1982, methods are described which utilize silicon and aluminum as pretreatments. In accordance with the procedure, the furnace tubes are pretreated with silicon and aluminum hours before introduction of the hydrocarbon feed stocks. With the use of silicon, furnace tubes are coated by the chemical vaporization of an alkoxysilane. While U.S. Pat. Nos. 4,105,540 and 4,116,812 are generally directed to fouling problems in general, the patents disclose the use of certain phosphate and phosphate and sulfur containing additives for use purportedly to reduce coke formation in addition to general foulants at high temperature processing conditions.
With respect to coke retardation in pyrolytic olefin production generally above 1400.degree. F., various efforts have been reported, namely:
1. French Pat. No. 2,202,930 (Chem. Abstracts Vol. 83, 30687K) is directed to tubular furnace cracking of hydrocarbons where molten oxides or salts of group III, IV or VIII metals (e.g., molten lead containing a mixture of K.sub.3 VO.sub.4, SiO.sub.2 and NiO) are added to a pretested charge of, for example, naphtha/steam at 932.degree. F. This treatment is stated as having reduced deposit and coke formation in the cracking section of the furnace. PA0 2. Starshov et al, Izv Vyssh. Uchebn. Zaved. Neft GAZ, 1977 (Chem. Abst. Vol. 87: 154474r) describes the pyrolysis of hydrocarbons in the presence of aqueous solutions of boric acid. Carbon deposits were minimized by this process. PA0 3. Nokonov et al., U.S.S.R. No. 834,107, 1981; (Chem. Abst. 95: 135651v) describes the pyrolytic production of olefins with peroxides present in a reactor, the internal surfaces of which have been pretreated with an aqueous alcoholic solution of boric acid. Coke formation is not mentioned in this patent since the function of the boric acid is to coat the inner surface of the reactor and thus decrease the scavenging of peroxide radicals by the reactor surface. PA0 4. Starshov et al., Neftekhimiya 1979 (Chem. Abst: 92: 8645j) describes the effect of certain elements including boron on coke formation during the pyrolysis of hydrocarbons to produce olefins.